1. Field of the Invention
The invention relates to a telephone set dial pulsing circuit having terminals for connection to the two terminals of a subscriber line, which dial pulsing circuit comprises for pulsed interruptions of the subscriber line a bipolar switching transistor whose main current path is situated between one of the line terminals and the remaining circuit elements of the telephone set and whose emitter is connected to the aforesaid terminal; in which the dial pulsing circuit also comprising a control circuit and a controllable current source connected to the base of the switching transistor, the current source output current increasing with the line current under control of the control circuit.
2. Description of the Related Art
Such a telephone set dial pulsing circuit is known from the U.S. Pat. No. 4,413,159.
It is generally desired in electronic telephone sets suitable for dial pulsing to include an electronic pulsing element, preferably a transistor. To provide that such a telephone set, if connected to a long subscriber line, will receive sufficient supply voltage, the pulsing element is required to have in the conducting state the least possible voltage drop across it.
This requirement of low voltage drop is met by bipolar transistors in the saturated state and by field effect transistors whose gate voltage by far exceeds the threshold voltage. In addition, such conductivity types should be chosen for these two types of transistors that the voltage of their control electrodes is constantly within the voltage existing across the two wires of the subscriber line. The latter requirement is because otherwise a step-up circuit is needed for driving the switching transistor, which is undesired in view of the required additional circuitry and the disturbing high frequency voltages at which such a circuit is to operate.
For the use as a switching transistor in an electronic telephone set which still has to operate even at relatively low line voltages, field effect transistors are less satisfactory than bipolar transistors. The control voltage of a field effect transistor differs considerably (for example 3.5 V) from the voltage of the source electrode, so that at a low DC line voltage (for example 4.2 V) the field effect transistor can be rendered non-conductive by an audio signal having an amplitude of for example 3 V and being superimposed on the DC line voltage.
Bipolar transistors meet the requirements of a low voltage drop and a low drive voltage. These transistors further meet the requirements that their control voltage lies between the voltages of the wires of the subscriber line, if conductivity types are chosen whose emitters can be connected (as regards the direction of conduction) to the telephone set terminals, that is to say to the incoming subscriber line.
In the aforementioned United States Patent a telephone circuit is shown in which the emitter of a bipolar switching transistor is connected to the incoming subscriber line. To keep this switching transistor at any magnitude of the line current in the saturated state without this transistor at a low line current-value deriving too much base current from the line current, this transistor is driven by means of an adjustable current source, linearly relating the base current of the switching transistor to the telephone line current. For this to be achieved the dial pulsing circuit comprises a current mirror circuit, connected in series between the switching transistor and the remaining elements of the telephone circuit. This current mirror circuit there causes a voltage loss of at least one diode forward voltage, so approximately 0.7 V. This takes place at the cost of the available supply voltage of the remaining elements of the telephone circuit, which is undesired.